Compensated DRAMA sequence for homonuclear dipolar recoupling under magic-angle spinning

The DRAMA sequence has been considered as the milestone in the development of homonuclear dipolar recoupling. Although it has a high efficiency for double-quantum excitation in spin 1/2 systems, it is seldom used today for real applications because of its susceptibility to the deteriorating effects of chemical shift anisotropy and resonance offsets. We show in this work that the practicability of DRAMA can be greatly enhanced by incorporating four π pulses with XY-4 phases into the basic DRAMA cycles. Average Hamiltonian theory is used to evaluate the performance of the resulting pulse sequence with respect to the compensation of chemical shift anisotropy. Numerical simulations and experimental measurements on hydroxyapatite indeed show that the performance of DRAMA-XY4 is very satisfying for 31P DQ excitation, provided that the resonance offset is within the range of [−4, 4] kHz.